1. The Field of the Invention
This invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to improvements in a free layer for a spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. The Relevant Technology
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe-Mn) layer.
The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1 shows a typical SV sensor 100 comprising a pair of end regions 104 and 106 separated by a central region 102. The central region 102 is formed by a suitable method such as sputtering and has defined end regions that are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 121.
The free layer 110, spacer 115, pinned layer 120 and AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current IS from a current source 160 to the MR sensor 100. A sensing device 170 connected to the leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor recently developed is an antiparallel (AP)-pinned spin valve sensor. FIG. 2 shows one representative AP-pinned SV sensor 200. The AP-pinned SV sensor 200 has a pair of end regions 202 and 204 separated from each other by a central region 206. The AP-pinned SV sensor 200 is also shown comprising a Ni-Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208 which is made of NiO.
The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 (PF1) of cobalt and a second ferromagnetic layer 216 (PF2) of cobalt separated from each other by a ruthenium (Ru) antiparallel coupling layer 214. The AFM layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. A pair of hard bias layers 235 and 240, formed in the end regions 202 and 204, provide longitudinal biasing for the free layer 225.
A pair of electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the SV sensor 200. In the depicted example, the magnetization direction of the free layer 225 is set parallel to the air bearing surface (ABS) in the absence of an external field. The magnetization directions of the pinned layers 212 and 214, respectively, are also set to be perpendicular to the ABS. The magnetization directions of the pinned layers are shown as coming out of the Figure at 260 and going in at 255. The magnetization of the free layer 225 is shown set to be parallel to the ABS.
The disk drive industry has been engaged in an ongoing effort to increase the GMR coefficient of the SV sensors in order to store more and more bits of information on any given disk surface. The issue is somewhat complicated, however, by the need to maintain a high degree of softness of the SV sensors, as measured by coefficients HC and HK. The softness of a SV sensor is a measurement of the threshold level of a magnetic field needed to change the magnetoresistance of a material by a given amount, typically that required to move the magnetic moment of the material from one orientation to another, offset from the first by 90xc2x0.
The softness of a material is referred to as its coercivity, HC, when the material exhibits domain walls, and more generally, by the property magnetocrystalline anisotropy, HK. Softness is increasingly important as disk drive densities increase and the magnetic field strengths of the recorded materials correspondingly decrease.
A further property exhibited by materials used in the formation of spin valves is magnetostriction. Magnetostriction is a measure of the stress or deformation of a material when it undergoes a change in magnetism. It is desired in the construction of spin valves to keep magnetostriction to a minimum.
Certain materials exhibiting a high degree of magnetoresistance have been found to exhibit a corresponding undesirably low degree of softness (i.e., an HK of 15 Oe or greater). Such materials include Co-Fe. It would be desirable to provide a spin valve sensor with a high degree of magnetoresistance (for instance, a change in resistance of about 7% or greater when a flat filament is exposed to a field of 50 Oe) through the use of materials such as Co-Fe, but which also maintains a high level of softness. It would also be desirable to provide such a spin valve sensor that maintains a low magnetostriction.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available spin valve sensors. Thus, it is an overall objective of the present invention to provide an improved spin valve sensor that overcomes some or all of the problems discussed above as existing in the art.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, a spin valve sensor with an improved free layer is provided. The spin valve sensor of the present invention in one embodiment comprises a free layer formed of a plurality of layers of a Co-Fe alloy and one or more intervening metal alloy layers containing at least Ni and Fe separating the plurality of layers of a Co-Fe alloy.
In alternate embodiments of the invention, the intervening metal alloy layers also comprise cobalt (Co). The intervening metal alloy layers may also comprise a metal selected from the group consisting of chromium, tantalum, rhodium, and molybdenum.
It is preferred that the layers of a Co-Fe alloy each have a thickness falling within a range of between about 2 Angstroms and about 15 Angstroms. A more preferred thickness is within a range of between about 5 Angstroms and about 10 Angstroms. Most preferably, the layers of a Co-Fe alloy each have a thickness of about 7 Angstroms. It is preferred that the free layer has a total thickness in a range of between about 20 xc3x85 and about 40 xc3x85, and that the layers of a Co-Fe alloy comprise at least 3 layers.
The layers of a Co-Fe alloy and the intervening metal alloy layers may, in one embodiment, comprise a first layer of a Co-Fe alloy; a first layer containing Ni and Fe; a second layer of a Co-Fe alloy; a second layer containing Ni and Fe; and a third layer of a Co-Fe alloy. In a currently preferred embodiment, the layers of a Co-Fe alloy may comprise Co90Fe10, and the layers Ni and Fe may comprise Ni81Fe19 or Ni66Fe16Co18.
In one further embodiment, the initial layer (closest to the spacer layer) of a Co-Fe alloy is substantially thicker than the subsequent layers of a Co-Fe alloy. For instance, an initial layer may have a thickness in a range of between 10 and 20 xc3x85 when the subsequent layers of a Co-Fe alloy have a thickness in a range of between about 2 and 10 xc3x85. In a more specific example, the initial layer may have a thickness of about 10 xc3x85, while the subsequent layers may have a thickness in a range of between about 5 xc3x85 to 7 xc3x85.
The spin valve sensor may comprise a cap layer; a free layer configured in the manner discussed above; a spacer layer; a pinned layer of ferromagnetic material; and an antiferromagnetic (AFM) layer. Nevertheless, the free layer of the present invention is intended for use with any type of spin valve sensor having any suitable construction.
The spin valve sensor of the present invention may be incorporated within a disk drive system comprising a magnetic recording disk; an anti-parallel (AP)-pinned spin valve (SV) sensor configured in the manner discussed above; an actuator for moving said spin valve sensor across the magnetic recording disk so the spin valve sensor may access different regions of magnetically recorded data on the magnetic recording disk; and a detector electrically coupled to the spin valve sensor for detecting changes in resistance of the sensor caused by rotation of the magnetization axis of the free ferromagnetic layer relative to the fixed magnetizations of the AP-pinned layer in response to magnetic fields from the magnetically recorded data.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.